The present invention relates generally to measuring the velocity spectrum of fluid traveling through a vessel and, more particularly to, a method and apparatus for tracking the motion of fluid and determining a velocity spectrum thereof from MR data acquired in a single cycle.
Determining and subsequent analyzing of flow characteristics of fluid flowing through a vessel have a number of applications, but are particularly applicable to the assessment of cardiovascular conditions. Quantitative measurements of blood motion may be used to assess the severity of vascular stenoses, the volumetric blood flow to tissues, and the mechanical properties of vessel walls. Known techniques implement non-invasive Doppler ultrasound to acquire quantitative measurements of blood through a vessel. Doppler ultrasound, however, has limited applicability because it is unable to access all vessels in some patients.
Other known techniques implement magnetic resonance (MR) technology to obtain measurements of blood motion. Current MR techniques have a number of advantages over Doppler ultrasound and include the ability to view entire vessels anywhere in the body, regardless of vessel angle, depth, and acoustic window. Notwithstanding the capability to interrogate vessels anywhere in the body, these known MR systems acquire measurements of blood motion very slowly and often require that data be combined from multiple cardiac cycles to provide usable measurements. Specifically, traditional MR techniques must often combine data from several cycles to improve the temporal resolution of the MR measurements.
Requiring data acquisition from several cardiac cycles to provide usable MR measurements of blood motion exposes the measurement process to the risk of gross patient motion often associated with longer scans that can have detrimental effects on image reconstruction. Further, over time, a periodic motion will introduce complex distortions to the MR measurements.
Several techniques have been developed to rapidly measure physiological motion using MR imaging processes. One such technique includes cardiac tagging to measure the motion and mechanical properties of the heart. Other known techniques have implemented tagging to visualize fluid flow. These known tagging techniques however, fail to accommodate signals produced by static tissue or signals produced by a range of blood velocities. Failure to recognize the impact of this velocity spectrum ultimately affects the accuracy of any blood displacement measurements acquired using tagging. Moreover, known cardiac tagging techniques are not able to provide accurate measurements of blood motion within a single heartbeat and at a high temporal resolution.
It would therefore be desirable to design a method and apparatus for tracking the motion of blood and determining its velocity spectrum from MR data collected within a single cardiac cycle. It would be further desirable to design a system for calculating a velocity spectrum that can be used to assess and/or remove any signals produced by static tissue to generate more accurate blood displacement measurements.
The present invention is directed to a method and system for tracking the motion of fluid traveling through a vessel and determining its velocity spectrum from magnetic resonance data collected within a single cycle. The present invention provides a method of independently acquiring and processing MR data from a single cardiac cycle. By combining a Spatial Modulation of Magnetization (SPAMM) excitation with a two-dimensional selective excitation positioned within a vessel, suitable tags of fluid flowing through the vessel may be created. This combination produces a sinusoidal variation of transverse magnetization along a column of the fluid. A succession of gradient echoes are then collected after the excitation to provide information about the flow of fluid in the excited vessel. Each gradient echo undergoes a transformation, i.e., such as Fourier transformation, to obtain a 1-D projection of the excited fluid across the vessel. The magnitude of each 1-D projection is then used to construct a velocity spectrum of the fluid flowing through the vessel. All of which overcomes the aforementioned drawbacks.
Therefore, in accordance with an aspect of the present invention, a method of measuring velocity of fluid flow in a vessel comprises the steps of identifying a vessel for fluid flow analysis and applying a pulse sequence to the vessel to excite fluid in the vessel. Application of the pulse sequence to the vessel further produces a sinusoidal variation of transverse magnetization along a column of fluid in the identified vessel. The method further includes the steps of acquiring a set of gradient echoes from the excited fluid where each gradient echo represents a 1-D projection of the excited fluid across the vessel and tracking the evolution of at least a portion of the 1-D projections. The method also includes the step of developing a velocity spectrum from the evolution tracking.
In accordance with a further aspect of the present invention, a computer program is provided that when executed by the computer causes the computer to apply a SPAMM excitation and cylindrical excitation to fluid flowing through a vessel. The computer is also programmed to acquire a set of gradient echoes from excited portions of fluid flowing through the vessel and determine a projection from each gradient echo. The computer program further causes the computer to organize the projections into a number of subsets where each subset represents a single projection and to modulate the projection of each subset to remove phase variations from the projections. The computer program further causes the computer to determine the magnitude of each projection and develop an evolutionary map representative of projection magnitude over time.
In accordance with yet a further aspect of the present invention, an MR apparatus includes an MRI system having a plurality of coils positioned about the bore of a magnet to impress a polarizing magnetic field. The MRI system further includes an RF transceiver system and an RF switch controlled by a pulse module to transmit and receive RF signals to and from a multi-coil RF coil assembly to acquire MR images. The MR apparatus further includes a computer programmed to apply a tagging sequence to a vessel to excite blood flowing through the vessel and to produce a sinusoidal variation of transverse magnetization along a region of blood in the vessel. The computer is further programmed to acquire a series of gradient echoes from the excited blood in a single cardiac cycle and transform each gradient echo to produce a corresponding 1-D projection. The computer is also programmed to analyze the 1-D projections to determine an evolutionary pattern of blood velocity.
Various other features, objects and advantages of the present invention will be made apparent from the following detailed description and the drawings.